Expandable vinyl chloride resin composition

ABSTRACT

An expandable vinyl chloride resin composition that the expansion ratio of a vinyl chloride resin can be greatly increased by the use of thermally decomposable inorganic blowing agents without the use of organic solvent blowing agents, and which comprises 100 parts by weight of a vinyl chloride resin, 5 to 30 parts by weight of, as a processing aid, a (meth)acrylic acid ester polymer having a specific viscosity of not less than 0.73 measured at 30° C. with respect to a solution of 0.1 g of the polymer dissolved in 100 ml of chloroform, and 2 to 25 parts by weight of a thermally decomposable inorganic blowing agent.

TECHNICAL FIELD

The present invention relates to an expandable vinyl chloride resincomposition, and more particularly to an expandable vinyl chloride resincomposition which is excellent in processability and provides cellularmoldings having a high expansion ratio and having excellent physicalproperties.

BACKGROUND ART

Polyvinyl chloride resins provide molded articles excellent in physicalproperties such as impact resistance and heat resistance and in chemicalproperties such as solvent resistance, acid resistance and alkaliresistance and, therefore, have been widely used in the field ofbuilding materials and other various fields. In recent years, foamingmethod attracts attention as a means for lightening of vinyl chlorideresins and cost reduction of molded articles. Molded articles of vinylchloride resins having a high expansion ratio have been demandedstrongly from the market.

In foaming of vinyl chloride resins, generally known is a method using ablowing agent in combination with a processing aid comprising methylmethacrylate as a main component.

It is known that it is possible to achieve foaming in a high expansionratio when an easily volatile organic solvent blowing agent such as analiphatic hydrocarbon or a halogenated aliphatic hydrocarbon is used asthe blowing agent mentioned above.

For example, Japanese Patent Publications Kokoku No. 60-10540 and KokokuNo. 58-40986 disclose that foamed articles having an expansion ratio ofseveral tens of times are obtained by impregnating a vinyl chlorideresin with an organic solvent having a boiling point of not more than90° C. such as butane or dichlorofluoromethane as a blowing agent or bydirectly introducing the organic solvent to an extruder during extrusionprocessing.

However, the use of an organic solvent blowing agent is disadvantageousin cost as compared with a thermally decomposable blowing agent, sinceequipment for the impregnation and for explosion proof is required incarrying out the molding.

On the other hand, in case of using a thermally decomposable blowingagent such as a thermally decomposable organic blowing agent or athermally decomposable inorganic blowing agent, it is difficult underexisting circumstances to raise the expansion ratio to more than about3-4 times so long as it is desired to produce foamed articles having asmooth surface and to keep the cells thereof uniform and fine.

For example, Japanese Patent Publication Kokoku No. 63-9540 discloses anexpandable vinyl chloride resin composition obtained by adding amethacrylate resin (polymethyl methacrylate having a degree ofpolymerization of 2,000 to 30,000, that is, a weight average molecularweight of 200,000 to 3,000,000) with a thermally decomposable organicblowing agent such as azodicarbonamide and a: thermally decomposableinorganic blowing agent such as sodium bicarbonate and further with afiller such as calcium carbonate to a vinyl chloride resin having anaverage degree of polymerization of 500 to 800. It is disclosed thatthis expandable vinyl chloride resin composition is molded to givefoamed articles having uniform and fine cells and: having excellentsurface property and surface hardness, but the expansion ratio is atmost about 3-4 times.

Also, Japanese Patent Publication Kokai No. 6-9813 discloses anexpandable vinyl chloride resin composition obtained by adding amethacrylate resin and a bicarbonate having a particle size of not morethan 10 μm as a thermally decomposable blowing agent to a vinyl chlorideresin. It is disclosed that this expandable vinyl chloride resincomposition is molded to give foamed articles having uniform and finecells and having good heat stability and weatherability, but no detailof the average molecular weight of methacrylate resin and the expansionratio is disclosed therein.

Further, Japanese Patent Publication Kokai No. 9-151269 discloses anexpandable vinyl chloride resin composition obtained by adding apolymethyl methacrylate resin having a weight average molecular weightof 4,500,000 to 7,000,000 and a thermally decomposable blowing agent toa vinyl chloride resin. It is disclosed that this expandable vinylchloride resin composition is molded to give injection-molded foamshaving uniform cells without occurrence of decomposition of the resins.However, in the working examples thereof, as the polymethyl methacrylateresin is used only P-531 made by Mitsubishi Rayon Co., Ltd. (weightaverage molecular weight 4,700,000) and no evaluation in the vicinity of7,000,000 in weight average molecular weight is made. Also, the obtainedexpansion ratios are at most about 2-3 times.

It is an object of the present invention to provide an expandable vinylchloride resin composition according to which the expansion ratio can bemarkedly increased by the use of a thermally decomposable inorganicblowing agent without using any organic solvent-based blowing agent inthe foaming.

A further object of the present invention is to provide an expandablevinyl chloride resin composition, which provides foamed articles havinga high expansion ratio and having uniform and fine cells.

DISCLOSURE OF INVENTION

The present inventors have found, as a result of intensive study, that acomposition capable of largely increasing the expansion ratio ascompared with conventional compositions is obtained by adding a specific(meth)acrylic acid ester polymer and a thermally decomposable inorganicblowing agent to a vinyl chloride resin.

In accordance with the present invention, there is provided anexpandable vinyl chloride resin composition comprising (a) 100 parts byweight of a vinyl chloride resin, (b) 5 to 30 parts by weight of, as aprocessing aid, a (meth)acrylic acid ester polymer having a specificviscosity of not less than 0.73 measured at 30° C. with respect to asolution of 0.1 g of the polymer dissolved in 100 ml of chloroform, and(c) 2 to 25 parts by weight of a thermally decomposable inorganicblowing agent.

Sodium bicarbonate is particularly preferred as the thermallydecomposable inorganic blowing agent.

The feature of the present invention resides in that a polymer which isobtained by emulsion polymerization of a monomer mixture containing apredominant amount of a methacrylic acid ester and/or an acrylic acidester and which has a high molecular weight is used as a processing aidfor vinyl chloride resins. By using the above-mentioned processing aidand the thermally decomposable inorganic blowing agent, the effect thatthe expansion ratio in the foaming can be increased without impairingexcellent physical and chemical properties that the vinyl chlorideresins originally possess can be remarkably exhibited with the additionof small amounts thereof.

BEST MODE FOR CARRYING OUT THE INVENTION

The vinyl chloride resins used in the present invention are notparticularly limited, and any of conventionally used vinyl chlorideresins can be used in the present invention. Homopolymer and copolymerscomposed of 80 to 100% by weight of units of vinyl chloride and 0 to 20%by weight of units of other monomers copolymerizable with vinyl chlorideare preferable.

Examples of the other monomer copolymerizable with vinyl chloride are,for instance, vinyl acetate, propylene, styrene, an acrylic acid ester(e.g., alkyl acrylates having a C₁ to C₈ alkyl group such as methylacrylate, ethyl acrylate, butyl acrylate and octyl acrylate), and othervinyl monomers. These may be used alone or in admixture thereof.

The average degree of polymerization of the vinyl chloride resin is notparticularly limited, but vinyl chloride resins having an average degreeof polymerization of about 400 to about 800 are usually employed.

Such vinyl chloride resins include, for instance, polyvinyl hloride,copolymers of not less than 80% by weight of units of vinyl chloride andnot more than 20% by weight of units of vinyl acetate, propylene,styrene or an acrylic acid ester, a post-chlorinated polyvinyl chloride,and the like. These may be used alone or in admixture thereof.

The processing aid is a component used for the purpose of improving theexpandability of the vinyl chloride resin.

In the present invention, as the processing aid are used a homopolymeror copolymers obtained by emulsion polymerization of a monomer mixture(hereinafter also referred to as “monomer mixture (M)”) containing apredominant amount of a methacrylic acid ester and/or an acrylic acidester. These polymers are important to have a high molecular weight, andthe specific viscosity thereof measured at 30° C. with respect to asolution of 0.1 g of the polymer dissolved in 100 ml of chloroform is atleast 0.73, preferably from 0.73 to 3, more preferably from 0.73 to 1.7,further preferably from 0.8 to 1.6, still further preferably from 0.9 to1.5. If the specific viscosity is less than 0.73, sufficientexpandability is not obtained. Also, if the specific viscosity exceeds1.7, the expandability tends to lower. The specific viscosity 0.73corresponds to about 7,300,000 in terms of weight average 20 molecularweight of the (meth)acrylic acid ester polymer.

The specific viscosity can be adjusted by means of the ratio ofpolymerization initiator to monomer or the amount of chain transferagent such as mercaptan.

The above-mentioned monomer mixture (M) is a mixture of 50 to 100% byweight of methyl methacrylate and 0 to 50% by weight of a monomerselected from an acrylic acid ester and a methacrylic acid esterexcepting methyl methacrylate, and it may further contain other vinylmonomers copolymerizable therewith.

Examples of the methacrylic acid ester excepting methyl methacrylateare, for instance, ethyl methacrylate, propyl methacrylate, butylmethacrylate, 2-ethylhexyl methacrylate and other alkyl 5 methacrylates.Examples of the acrylic acid ester are, for instance, methyl acrylate,ethyl acrylate, propyl acrylate, butyl acrylate, 2-ethylhexyl acrylateand other alkyl acrylates. These acrylates and methacrylates exceptingmethyl methacrylate may be used alone or in admixture thereof.

Examples of the other vinyl monomer are, for instance, an aromatic vinylcompound such as styrene or α-methylstyrene, an unsaturated nitrilecompound such as acrylonitrile, and the like. These may be used alone orin admixture thereof.

The proportions of the respective components in the monomer mixture (M)are from 50 to 100% by weight, preferably 50 to 90% by weight, morepreferably 60 to 85% by weight, of methyl methacrylate, and from 0 to50% by weight, preferably 10 to 50% by weight, more preferably 15 to 40%by weight, of a monomer selected from the methacrylic acid esterexcepting methyl methacrylate and the acrylic acid ester, and from 0 to20% by weight, preferably 0 to 10% by weight, more preferably 0 to 5% byweight, of the other copolymerizable vinyl monomer.

If the proportion of methyl methacryalte in the monomer mixture (M) isless than 50% by weight, the processability and expandability arelowered. If the proportion of the monomer selected from an acrylic acidester and a methacrylic acid ester excepting methyl methacrylate exceeds50% by weight, the processability and expandability tend to be lowered.Further, if the proportion of the other vinyl monomer exceeds 20% byweight, the gelling property and expandability tend to be lowered.

The above-mentioned processing aid is obtained, for example, byemulsion-polymerizing the monomer mixture (M), using suitable dispersionmedium, emulsifier, polymerization initiator and chain transfer agent,in a known manner. The emulsion polymerization may be carried out in twostages or more, or may be carried out in a single stage.

The dispersion medium used in the emulsion polymerization is usuallywater.

Known emulsifiers are used. Examples of the emulsifier are, forinstance, an anionic surfactant such as a: fatty acid salt, an alkylsulfate, an alkylbenzene sulfonate, an alkyl phosphate or asulfosuccinic acid diester, and a non-ionic surfactant such as apolyoxyethylene alkyl ether or a polyoxyethylene fatty acid ester.

As the polymerization initiator are :used water-soluble and oil-solublepolymerization initiators. For example, usual inorganic polymerizationinitiators such as a persulfate, organic peroxides or azo compounds maybe used alone, or may be used as a redox system in combination with asulfite, a thiosulfate, a primary metal salt, formaldehyde sodiumsulfoxylate or the like. Preferable persulfates include, for instance,sodium persulfate, potassium persulfate, ammonium persulfate, and thelike. Preferable peroxides include, for instance, t-butyl hydroperoxide,cumene hydroperoxide, benzoyl peroxide, lauroyl peroxide, and the like.

The chain transfer agent is not particularly limited. For instance,t-dodecylmercaptan, t-decylmercaptan, n-dodecylmercaptan,n-decylmercaptan and the like can be used.

The temperature and time of the emulsion polymerization reaction are notparticularly limited and are suitably adjusted so as to obtain thedesired specific viscosity and particle size in accordance with thepurpose of use.

In case of carrying out the emulsion polymerization in two or morestages, the monomer of the next stage is added after confirming that theprior stage polymerization is completed, whereby the polymerization ineach stage can be carried out without mixing of the next stage monomerwith the prior stage monomer.

The particles in the thus obtained polymer latex have usually an averageparticle size of about 100 to about 3,000 Å(0.01 to 0.3 μm). Theparticles are taken out of the latex by salting out or coagulation bymeans of addition of a usual electrolyte, or spray drying in hot air.Also, as occasion demands, washing, dehydration, drying and the like arecarried out in a usual manner.

In general, the processing aid obtained by the above-mentionedprocedures is preferably a white powder having an average particle sizeof 30 to 300 μm from the viewpoint of incorporating it as a processingaid into a vinyl chloride resin or an expandable vinyl chloride resincomposition.

The processing aid is used in an amount of 5 to 30 parts by weight,preferably 5 to 25 parts by weight, more preferably 8 to 25 parts byweight, per 100 parts by weight of a vinyl chloride resin. If the amountof the processing aid is less than 5 parts by weight, the effect to beproduced by the addition of the processing aid is not sufficientlyobtained. If the amount is more than 30 parts by weight, excellentmechanical properties of the vinyl chloride resin are impaired.

Examples of the thermally decomposable inorganic blowing agent used inthe present invention are, for instance, sodium bicarbonate, potassiumbicarbonate, ammonium bicarbonate, sodium carbonate, ammonium carbonate,and the like. These may be used alone or in admixture thereof. Of these,sodium bicarbonate is preferred from the viewpoints of expansionefficiency and cost.

The amount of the thermally decomposable inorganic blowing agent isselected in accordance with the purpose without particular restriction.The amount is usually from 2 to 25 parts by weight, especially from 3 to18 parts by weight, per 100 parts by weight of a vinyl chloride resin.If the amount of the blowing agent is less than 2 parts by weight,foamed articles having a sufficient expansion ratio are not obtained. Ifthe amount is more than 25 parts by weight, foamed articles having auniform cell are hard to be obtained.

Also, the above-mentioned amount of the thermally decomposable inorganicblowing agent may be changed in conformity with the amount of theprocessing aid. For example, it is preferable to use the blowing agentin an amount of, per 100 parts by weight of a vinyl chloride resin, 3 to8 parts by weight when the processing aid is used in an amount of 10parts by weight, and in an amount of 4 to 18 parts by weight when theprocessing aid is used in an: amount of 20 parts by weight.

The expandable vinyl chloride resin composition of the present inventionmay contain one or more of other additives such as stabilizer,lubricant, impact modifier, plasticizer, colorant, filler and the like,as occasion demands.

The process for preparing the expandable vinyl chloride resincomposition of the present invention is not particularly limited. Forexample, the composition can be prepared in such a manner as mixing avinyl chloride resin, the processing aid, a thermally decomposableinorganic blowing agent and other additives, and melt-kneading themixture at an appropriate temperature by a melt kneader such as a twinscrew extruder.

The method of the molding processing of the expandable vinyl chlorideresin composition of the present invention is not particularly limited,and generally used molding methods such as extrusion are applicable.

The present invention is more specifically explained by means ofexamples and comparative examples, in which all parts and % are byweight unless otherwise noted. It is to be understood that the presentinvention is not limited to these examples.

Methods of evaluation used in the examples and comparative examples areshown below. (Measurement of specific viscosity of polymer sample(processing aid))

In 100 ml of chloroform was dissolved 0.1 g of a polymer sample, and themeasurement was carried out using a Ubbelohde's viscometer kept at aconstant temperature in a water bath of 30° C. (Measurement of expansionratio)

After measuring the specific gravity of a obtained powder compound(non-foamed molding of vinyl chloride resin composition), the compoundwas molded by a small-size single screw extruder attached to aLaboplasto mill made by Toyo Seiki Kabushiki Kaisha to give a rod-likefoamed article (foamed molding of vinyl chloride resin composition), andthe specific gravity of the rod-like foamed article was measured. Theexpansion ratio was calculated from the measured values according to thefollowing equation.

Expansion ratio =(specific gravity of non-foamed molding)/(specificgravity of foamed molding)

The specifications of extruder and the molding conditions are shownbelow.

Specifications of extruder

Screw: L/D=20, compression ratio=2.7, number of rotation=30 r.p.m.

Die: diameter 5 mm, 1 and=20 mm

Molding conditions

Molding temperature: C1=170° C., C2=175° C., C3=180° C. Die=185° C.

(Measurement of Polymerization Conversion)

The polymerization conversion was calculated according to the followingequation.

Polymerization conversion (%)=(amount of produced polymer/amount ofmonomers charged)×100

(Measurement of Average Particle Size of Latex)

With respect to a latex obtained, the average particle size was measuredusing light scattering in a wavelength of 546 nm by a SpectrophotometerU-2000 made by Hitachi, Ltd.

(Evaluation of Cell State)

With respect to the cell state of a foamed article obtained, theappearance was visually observed and evaluated according to thefollowing criteria.

◯: The cell structure is uniform and the appearance is excellent.

Δ: Broken cells are scattered.

X: Most of cells are broken and the appearance is poor.

EXAMPLE 1

An 8 liter reactor equipped with a stirrer was charged with 0.7 part ofsodium dioctylsuccinate dissolved in water as an emulsifier, and theretowas added water so that the total amount of water including waterincluded in sub-raw materials added later became 200 parts. Afterpassing a nitrogen gas through the gaseous phase and liquid phase of thereactor to expel oxygen from the space and water, the temperature of thecontent was elevated to 70° C. with stirring. To the reactor was thenadded at a time a first stage monomer mixture composed of 68 parts ofmethyl methacrylate (hereinafter also referred to as “MMA”) and 12 partsof butyl acrylate (hereinafter also referred to as “BA”). Subsequently0.01 part of potassium persulfate was added as an initiator, andstirring was continued for 1 hour to substantially complete thepolymerization. A second stage monomer mixture composed of 6 parts ofMMA and 14 parts of BA was added dropwise at a rate of about 30 partsper hour. After the completion of the dropwise addition, the content inthe reactor was kept at 70° C. for 90 minutes and was then cooled togive a latex. The average particle size of the latex was measured. Theresult is shown in Table 1.

The polymerization conversion was 99.5%. The latex was coagulated bysalting out with an aqueous solution of calcium chloride, heat-treatedby elevating the temperature to 90° C. and dehydrated by a cetrifugaldehydrator. The obtained cake of a resin was washed with anapproximately same amount of water as the weight of the resin and wasdried at 50° C. for 15 hours by a parallel flow dryer to give a whitepowder of polymer sample (1). The specific viscosity of the obtainedpolymer sample (1) was measured. The result is shown in Table 1.

In a Henschel mixer, 100 parts of a polyvinyl chloride (KANEVINYL S-1007made by Kaneka Corporation, average degree of polymerization 680) wasmixed with 10.0 parts of the above polymer sample (1), 6.0 parts ofcalcium carbonate, 2.0 parts of titanium oxide, 2.0 parts of an octyltin mercapto-type stabilizer [di-n-octyl tin bis(iso-octylmercaptoacetate)] (TVS #8831 made by Nitto Kasei Kabushiki Kaisha), 0.6part of calcium stearate, 0.1 part of hydroxystearic acid (LOXIOL G-21made by Henkel GmbH), 0.9 part of a dibasic fatty acid alcohol ester(LOXIOL G-60 made by Henkel GmbH) and 0.6 part of a polyethylene wax(ACPE-617A made by Allied Chemical Corporation), and the innertemperature was elevated to 110° C. After cooling, 6.0 parts of sodiumbicarbonate was incorporated into the mixture to give a powder compound.The compound was molded by a small-size extruder attached to aLaboplasto mill made by Toyo Seiki Kabushiki Kaisha to give a roundrod-like foamed article. The cell state of the foamed article wasevaluated, and the expansion ratio was measured. The results are shownin Table 1.

EXAMPLES 2 to 4 and COMPARATIVE EXAMPLES 1 and 2

Polymer samples (2) to (6) were prepared according to the recipe shownin Table 1 in the same manner as in Example 1, and the characteristicsthereof were measured. Also, round rod-like foamed articles wereobtained by incorporating each of the obtained polymer samples (2) to(6) into polyvinyl chloride in the same manner as in Example 1, and wereevaluated. The results are shown in Table 1.

TABLE 1 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Com. Ex. 1 Com. Ex. 2 Polymer sample No.(1) (2) (3) (4) (5) (6) Composition (part) Polymer sample 1st stagemixture MMA 68 68 68 68 68 68 BA 12 12 12 12 12 12 2nd stage mixture MMA6 6 6 6 6 6 BA 14 14 14 14 14 14 Initiator 0.01 0.007 0.003 0.001 0.030.1 Emulsifier 0.7 0.7 0.7 0.7 0.5 0.5 Results of evaluationPolymerization conversion (%) 99.5 99.7 99.5 99.4 99.2 99.4 Specificviscosity of polymer sample 0.73 0.82 0.92 1.06 0.60 0.33 Averageparticle size of latex (μm) 0.15 0.16 0.16 0.16 0.16 0.16 ExpandabilityExpansion ratio (times) 7.8 8.2 10.1 11.8 3.9 2.8 Cell state ∘ ∘ ∘ ∘ Δ Δ

From the results shown in Table 1, it is understood that compositionshaving a good expandability are obtained when polymer samples (1) to (4)having a specific viscosity of not less than 0.73, but no sufficientexpandability is obtained when polymer samples (5) and (6) having aspecific viscosity of less than 0.73, and accordingly the specificviscosity of the polymer is required to be not less than 0.73 forobtaining the effects of the present invention.

EXAMPLES 5 to 7 and COMPARATIVE EXAMPLES 3 and 4

Foamed articles were prepared in the same manner as in Example 3 exceptthat the amount of polymer sample (3) was changed as shown in Table 2instead of 10.0 parts per 100 parts of polyvinyl chloride in order toevaluate a change in expandability when the amount of polymer sample (3)incorporated into polyvinyl chloride was changed. The expandability ofthe foamed articles was evaluated. The results are shown in Table 2,provided that in Comparative Example 4 a foamed article suitable toevaluate the expandability could not be obtained due to non-uniformityof a composition obtained.

TABLE 2 Com. Com. Ex. 3 Ex. 5 Ex. 6 Ex. 7 Ex. 3 Ex. 4 Polymer sample No.(3) (3) (3) (3) (3) (3) Amount (part) 10 8 15 20 3 40 ExpandabilityExpansion ratio (times) 10.1 7.1 11.1 13.4 3.8 — Cellstate ◯ ◯ ◯ ◯ ◯ —

From the results shown in Table 2, it is understood that thecompositions incorporated with the processing aid according to thepresent invention exhibit a good expandability, but sufficientexpandability is not obtained if the amount of the processing aid issmall as shown in Comparative Example 3.

EXAMPLES 8 to 15 and COMPARATIVE EXAMPLES 5 to 9

Foamed articles were prepared in the same manner as in Example 3 exceptthat the amount of polymer sample (3) and the kind and amount of theblowing agent were changed as shown in Table 3 in order to evaluate achange in expandability based on the amount of processing aidincorporated into polyvinyl chloride and the kind and amount ofthermally decomposable blowing agent. The expandability of the foamedarticles was evaluated. The results are shown in Table 3.

In the table, SBC denotes sodium bicarbonate, and ADCA denotesazodicarbonamide.

TABLE 3 Com. Com. Com. Com. Com. Example No. 8 9 10 11 12 13 14 15 Ex. 5Ex. 6 Ex. 7 Ex. 8 Ex. 9 Polymer sample Number (3) (3) (3) (3) (3) (3)(3) (3) (3) (3) (3) (3) (3) Amount (part) 10 10 10 10 20 20 20 20 10 2010 10 10 Blowing agent Kind SBC SBC SBC SBC SBC SBC SBC SBC SBC SBC ADCAADCA ADCA Amount (part) 3 5 7 8 4 10 15 20 1.5 27 1 2 3 ExpandabilityExpansion ratio (times) 5.2 7.8 10.0 8.2 9.1 13.8 14.0 10.5 3.8 3.9 3.03.6 2.3 Cell state ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ x ∘ ∘ x

From the results shown in Table 3, it is understood that as shown inComparative Examples 5 and 6, a high expansion ratio is not obtained ifthe amount of a thermally decomposable inorganic blowing agent is small,and foamed articles having uniform cells are not obtained if the amountis too large. It is also understood that a sufficient expandability isnot obtained also in the case that thermally decomposable blowing agentsother than a thermally decomposable inorganic blowing agent are used, asshown in Comparative Examples 7 to 9.

INDUSTRIAL APPLICABILITY

The expandable vinyl chloride resin composition of the present inventionprovides foamed articles having an expansion ratio as high as 5-10 ormore in spite of the use of thermally decomposable inorganic blowingagents and, moreover, having a good state of cells. Therefore, costreduction is possible since an existing extruder can be used, and alsothe range of uses is expanded.

What is claimed is:
 1. An expandable vinyl chloride resin compositioncomprising 100 parts by weight of a vinyl chloride resin, 5 to 30 partsby weight of, as a processing aid, a (meth)acrylic acid ester polymerhaving a specific viscosity of not less than 0.73 measured at 30° C.with respect to a solution of 0.1 g of the polymer dissolved in 100 mlof chloroform, and 2 to 25 parts by weight of a thermally decomposableinorganic blowing agent, said composition containing no organic solventblowing agent, wherein the resultant expandability ratio is not lessthan 5:1, expandability being defined as (specific gravity of non-foamedmolding)/(specific gravity of foamed molding).
 2. The composition ofclaim 1, wherein said thermally decomposable inorganic blowing agent issodium bicarbonate.
 3. The composition of claim 1, wherein said(meth)acrylic acid ester polymer has a specific viscosity of 0.8 to 1.6.4. The composition of claim 1, wherein said (meth)acrylic acid esterpolymer is a polymer of 50 to 100% by weight of methyl methacrylate, 50to 0% by weight of a monomer selected from acrylic acid esters andmethacrylic acid esters excepting methyl methacrylate and 20 to 0% byweight of other vinyl monomer copolymerizable therewith.
 5. Thecomposition of claim 4, wherein said other vinyl monomer is a memberselected from the group consisting of aromatic vinyl compounds andunsaturated nitrile compounds.
 6. The composition of claim 1, whereinsaid (meth)acrylic acid ester polymer is a copolymer of 50 to 90% byweight of methyl methacrylate and 50 to 10% by weight of a monomerselected from the group consisting of ethyl methacrylate, propylmethacrylate, butyl methacrylate, 2-ethylhexyl methacrylate, methylacrylate, ethyl acrylate, propyl acrylate, butyl acrylate and2-ethylhexyl acrylate.
 7. The composition of claim 1, wherein theresultant expandability ratio is not less than 5.2: 1, expandabilitybeing defined by (specific gravity of non-foamed molding)/(specificgravity of foamed molding).
 8. The composition of claim 1, wherein theresultant expandability ratio is not less than 7.1: 1, expandabilitybeing defined by (specific gravity of non-foamed molding)/(specificgravity of foamed molding).
 9. A foamed molding obtained by molding thecomposition of claim 1 by extrusion.
 10. The composition of claim 1,wherein the amount of said thermally decomposable inorganic blowingagent is from 6 to 25 parts by weight per 100 parts by weight of saidvinyl chloride resin.